This invention relates to a magnetic resonance imaging system (hereinafter called "MRI"), and more particularly to a short head coil apparatus for imaging the human head or brain.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are superconducting, such that the coils can be operated in persistent mode, that is, a power supply can be connected for a short time to start current flowing through the coils, then a superconducting switch can be closed, the power supply removed, and the current will continue to flow, thereby maintaining the coil current and resultant magnetic field. Superconducting magnets find wide application in the field of MRI.
In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrical cryogen pressure vessel, contained within a vacuum vessel and forming an imaging bore in the central region. The main magnet coils develop a strong magnetic field in the imaging bore.
However, it is necessary for acceptable quality imaging to provide and maintain a strong homogeneous magnetic field in the imaging region. Typical field strengths of 0.5 to 1.5 T (Tesla) require homogeneity of 10parts per million (ppm) over volumes on the order of a 45 cm diameter sphere.
In MRI imaging a plurality of additional magnetic fields are added in order to shape the imaging magnetic field and to conduct MRI operation. These include radio frequency (hereinafter called "RF") signals and include pulsed magnetic field gradients to spatially encode the nuclear magnetic resonance (NMR) signal from various portions of an anatomical region of interest. The pulsed magnetic field gradients together with RF excitation of the nuclear spins. and acquisition of signal information are commonly referred to as pulse sequences. Pulsing current through conductors generate pulsing magnetic fields external to the conductors which can interfere with other magnetic fields in the superconducting magnet including the homogeneity of the main magnetic field in the imaging volume, and adversely affect imaging quality.
It is known in the imaging of the human head and brain to utilize a head coil or additional coil assembly, positioned around the head during imaging. The separated parallel conductors of such head coils has led to their being called birdcages.
RF head coils are applied to generate a uniform RF magnetic field over the brain. In general these head coils are very long, and continue past the apex of the patient's head. Since the signal to noise ratio is proportional to the square root of the Quality factor of the loaded coil divided by the effective volume, which is a volume integral of the RF energy stored in the coil, it is important to limit the size of RF coils and keep them as small as possible while at the same time providing the necessary MRI signals. However, this has to be done while maintaining the RF magnetic field homogeneity of the coils for equal flip angle distribution during transmit pulses and equal sensitivity during receive.
Attempts to shorten the RF head coil have not proven entirely satisfactory, providing for example hot spots or areas of high B1 amplitude which disrupt the RF magnetic field homogeneity and the imaging quality.